Graphite is a form of crystalline carbon. The carbon atoms within graphite are densely arranged in parallel-stacked, planar, honeycomb-lattice sheets. Graphite is a soft mineral which exhibits perfect basal cleavage. It is flexible but not elastic, has a low specific gravity, is highly refractory, and has a melting point of 3,927° C. Of the non-metals, graphite is the most thermally and electrically conductive, and it is chemically inert. These properties make graphite beneficial for numerous applications in a range of fields.
Worldwide demand for graphite has increased in recent years, and is expected to continue to increase as global economic conditions improve and more graphite-using non-carbon energy applications are developed.
Some examples of the uses of graphite include use as a steel component, static and dynamic seals, low-current, long-life batteries (particularly lithium ion batteries), rubber, powder metallurgy, porosity-enhancing inert fillers, valve and stem packing, and solid carbon shapes. Graphite is also used in the manufacture of supercapacitors and ultracapacitors, catalyst supports, antistatic plastics, electromagnetic interference shielding, electrostatic paint and powder coatings, conductive plastics and rubbers, high-voltage power cable conductive shields, semiconductive cable compounds, and membrane switches and resistors.
In recent years, graphite has been important in the emerging non-carbon energy sector, and it has been used in several new energy applications such as in pebbles for modular nuclear reactors and in high-strength composites for wind, tide and wave turbines. Graphite has also been used in energy storage applications such as bipolar plates for fuel cells and flow batteries, anodes for lithium-ion batteries, electrodes for supercapacitors, phase change heat storage, solar boilers, and high-strength composites for flywheels. Furthermore, graphite is used in energy management applications such as high-performance polystyrene thermal insulation and silicon heat dissipation. The new and increasing demand from these energy applications may require double the current graphite supply when fully implemented, and current graphite capacity may not be adequate to meet this rising demand.
U.S. production of synthetic graphite in 2011 was estimated to be about 148,000 metric tons valued at over $1 billion.
The current dominant approach for producing synthetic graphite is a time-consuming multi-step process including the phases of (1) powder preparation, (2) shape forming, (3) baking, and (4) graphitization.
In the powder preparation step, raw materials such as petroleum coke are pulverized in crushers and ball mills. The resulting powder is conditioned according to the particle size distribution using screening or sieving. Petroleum coke usually contains about 10-20% volatile components such as water and other volatile organic matter which must be removed before the petroleum coke is suitable for manufacturing graphite. These volatile components are removed through the calcining process, which involves heating the coke to a sufficiently high temperature to volatize, vaporize, or burn off all volatile components. The sieved, calcined powder is then blended with a binder such as coal tar pitch, petroleum pitch or synthetic resins.
In the shape forming step, the carbon powder mixed with binder is compacted through a shape forming technique such as cold isostatic pressing, extrusion or die molding. Cold isostatic pressing involves applying pressure from multiple directions through a liquid medium surrounding the compacted part, and the process generally takes place at room temperature. Extrusion involves forcing the mixture through a die with an opening, resulting in a long product with a regular cross-section such as rods, bars, long plates and pipes, which can be cut to the desired length. Die molding involves placing the powder in a die between two rigid punches and applying uniaxial pressure to the powder to compact it.
During the baking step, the compacted parts are heat treated in a baking furnace at 1000-1200° C. for 1-2 months in the absence of air. The baking process is also known as carbonization, and it results in the thermal decomposition of the binder into elementary carbon and volatile components.
Lastly, in the graphitization step, the baked parts are heat treated in an Acheson furnace at 2500-3000° C. The high temperatures present in this step require the exclusion of oxygen from the furnace, accomplished by covering the carbon particles with some type of oxygen scavenging material such as petroleum coke or metallurgical coke. The graphitization step generally takes two to three weeks, resulting in a typical overall processing time of around 1.5-3 months for the conventional method of producing synthetic graphite.
The quality of the graphite produced through this conventional method is correlated with the quality of the petroleum coke feedstock, whose price is linked to the price of oil.